1. Field of the Invention
The present invention relates to a ferroelectric ceramic material, in particular, a non-lead-based ferroelectric ceramic material of high performance capable of controlling a Curie temperature (Tc).
2. Description of the Related Art
Conventionally, as a ceramic material, a perovskite (ABO3)-type oxide has been used as a ceramic capacitor material or a ceramic piezoelectric material. In particular, in a ferroelectric ceramic material, due to the emergence of a lead zirconate titanate (PZT), the PZT has become a mainstream of a piezoelectric ceramic material for a piezoelectric actuator of an inkjet head, a piezoelectric ultrasonic motor, and the like. A PZT ferroelectric material has also been used in FeRAM using its residual dielectric polarization. However, recently, lead has been considered to cause a problem as one of contaminants, and hence, there is an increasing demand for the development of a non-lead-based ferroelectric ceramic material containing no lead.
As the non-lead-based ferroelectric ceramic material, barium titanate (BaTiO3) is well-known. BaTiO3 is less expensive than a Nb-based material. However, due to the presence of transitions at room temperature (hereinafter, referred to as “Tr”) from tetragonal to orthorhombic systems in the vicinity of 5° C., the ferroelectric properties of BaTiO3 change in the vicinity of Tr, which makes it difficult to use BaTiO3 in a number of applications. As a method of suppressing Tr of BaTiO3, a method of replacing Ba with Ca is known. Further, although the Curie temperature of BaTiO3 is in the vicinity of 125° C., when a solid solution is formed from BaTiO3 and a number of perovskite-type oxides, the Curie temperature (Tc) tends to decrease rapidly, resulting in loss of ferroelectricity in the vicinity of a room temperature.
Further, it is also known that, when another metal oxide is added to BaTiO3, the characteristics of BaTiO3 change considerably. For example, Japanese Patent Application Laid-Open No. 2007-290940 discloses a dielectric ceramic capable of being calcined at a low temperature, in which CuO, MgO, ZnO, or Bi2O3 is added to BaTiO3 in an amount of 4 to 10% by weight. Japanese Patent Application Laid-Open No. H03-45559 discloses a barium titanate ceramic exhibiting varistor nature, in which a rare earth oxide, a transition metal oxide, Bi2O3, and the like are added to BaTiO3. Japanese Patent Application Laid-Open No. H02-225371 discloses a high-capacitance capacitor obtained by calcinating a layer of a dielectric composition formed of BaTiO3, 8 to 0.5% by weight of a fluoride of Pb and Li, 4 to 0% by weight of an oxide of Bi, Zn, and Pb, and 8 to 1% by weight of an assistant. However, these ceramics are paraelectric and have low insulation property. An effective method of suppressing the phase transition of BaTiO3 while keeping ferroelectricity has not been disclosed.
As described above, as a non-lead-based ferroelectric ceramic material, a ferroelectric ceramic material containing BaTiO3 as a main component capable of preventing the decrease in the Curie temperature while suppressing the phase transition has not been known.
The present invention has been achieved in view of the above-mentioned background art, and an object thereof is to provide a ferroelectric ceramic material containing BaTiO3 as a main component, capable of suppressing Tr of BaTiO3 without decreasing the Curie temperature substantially.